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To J. D. Hooker 31 October 1873

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Oct 31 1873

My dear Hooker

I will now answer the rest of yr letter; but first, I forgot to ask what temp. A. Farn: (if sent) requires?1

With Nepenthes I wd certainly try a little infusion of raw meat, & mechanical irritation to see if it will not increase the acid in a virgin pitcher.2 Thanks for the news about the Trachææ & glands.3 I am extremely glad to hear that you mean to attack Cephalotus &c.4 You will make, I believe, a splendid paper. Now I am going to shew what you will think a very petty feeling; but when you publish, do just say that I had previously observed the digestive power in Drosera, & about aggregation.5 Otherwise botanists will say that I feloniously took it all from you, & I shall be perplexed how to give my facts.

With respect to Earl Russell I shd feel honoured & pleased by calling on him, but I doubt my strength unless he is in London.6 I shd so like to come down to Kew, & will see whether I can;—but hitherto it has always knocked me up. Thanks for the sight of yr immersion lens; I have not yet tried it, but will bring it back with me to London.7

Now I want to tell you, for my own pleasure, about the movements of Desmodium.8

(1) When the plant goes to sleep, the terminal leaflets hang vertically down, but the petioles move up towards the axis, so that the dependent leaves are all crowded round it. The little leaflets never go to sleep, & this seems to me very odd; they are at their games of play as late as 11 o’clock at night & probably later.

(2) If the plant is shaken or syringed with tepid water, the terminal leaflets move down thro’ about an angle of 45o; & the petioles likewise move about 11o downwards; so that they move in an opposite direction to what they do when they go to sleep. Cold water or air produces the same effect as does shaking. The little leaflets are not in the least affected by the plant being shaken or syringed.— I have no doubt from various facts, that the downward movement of the terminal leaflets & petioles from shaking & syringing is to save them from injury from warm rain.

(3) The axis, the main petiole & the terminal leaflets are all, when the temp. is high, in constant movement, just like that of climbing plants. This movement seems to be of no service, any more than the incessant movement of amœboid bodies The movement of the terminal leaflet, tho’ insensible to the eye, is exactly the same as that of the little lateral leaflets, viz from side to side, up & down, & half-round their own axes.

The only difference is that the little leaflets move to a much greater extent & perhaps more rapidly; & they are excited into movement by warm water, which is not the case with the terminal leaflet. Why the little leaflets, which are rudimentary in size & have lost their sleep-movements & their movements from being shaken, shd not only have retained, but have their spontaneous movements exaggerated, I cannot conceive. It is hardly credible that it is a case of compensation. All this makes me very anxious to examine some plant (if possible one of the Leguminosae) with either the terminal or lateral leaflets greatly reduced in size, in comparison with the other leaflets on the same leaf. Can you or any of yr colleagues think of any such plant? It is indirectly on this account that I so much want the seeds of Lathyrus Nissolia.9

I hear from Frank that you think that the absence of both lateral leaflets, or of one alone, is due to their having dropped off; I thought so at first, & examined extremely young leaves from the tips of the shoots, & some of them presented the same characters.10 Some appearances make me think that they abort by becoming confluent with the main petiole.

I hear also that you doubt about the little leaflets ever standing not opposite to each other: pray look at the enclosed old leaf, which has been for a time in spirits, & can you call the little leaflets opposite? I have seen many such cases on both my plants, tho’ few so well marked.

I hope this letter will not bore you | yours affectionately | Ch. Darwin

Hooker mentioned that the pouches containing the glands of Nepenthes pointed downwards, which might support CD’s view of the glands as secreting organs, in his letter of 29 October 1873 (see also J. D. Hooker 1874, p. 112). ‘Trachaeae’ may be a reference to the spiral vessels also mentioned in this letter (tracheae are conducting tubes in the xylem of trees and plants).

CD later described the digestive powers of Drosera rotundifolia (the common or round-leaved sundew), and the aggregation of the protoplasm within the cells of the tentacles (Insectivorous plants, pp. 38–65 and 85–135).